There has been a wide proliferation of secondary batteries not only as power sources for portable devices such as mobile phones, digital cameras, and laptop computers, but also as power sources for vehicles and households. In particular, a lightweight lithium ion secondary battery with a high-energy density is an energy storage device that has become indispensable for daily life.
The secondary battery can be roughly categorized as a wound type or a laminated type. An electrode element of the wound-type secondary battery has a structure in which a long positive electrode sheet and a long negative electrode sheet are wound multiple times in overlapping relation with a separator interposed between each positive and negative electrode sheet. An electrode element of the laminated-type battery has a structure in which positive electrode sheets and negative electrode sheets are laminated alternately and repeatedly with separators respectively interposed therebetween. The positive electrode sheet and the negative electrode sheet each include an application portion (coating portion) that is formed by applying active material slurry (including a case of a mixture agent including a binding agent, a conductive material and the like in addition to the active material) on a current collector and drying it, and also include a non-application portion (non-coating portion) where the active material is not applied for the connection with an electrode terminal.
In each of the wound-type secondary battery and the laminated-type secondary battery, the battery electrode assembly is contained and sealed in an outer container (outer case), such that one end of a positive electrode terminal and one end of a negative electrode terminal are electrically connected with the non-application portion of the positive electrode sheet and the non-application portion of the negative electrode sheet respectively, and the other end of the positive electrode terminal and the other end of the negative electrode terminal extend from the outer container. Together with the battery electrode assembly an electrolyte is contained and sealed in the outer container. With yearly improvements in battery technology, the trend is for annual increases in the capacity of secondary batteries and for annual increases in the thickness of the electrode. What this means is that, should a short circuit occur, the amount of generated heat will increase which, in turn, increases safety risks. Therefore, measures to improve battery safety become more and more important.
In a lithium ion secondary battery described in Patent Document 1, during charge and discharge, the occlusion and release of lithium ions are performed between the positive electrode and the negative electrode that face each other. Then, it is known that the ratio between charging capacity A of the negative electrode and charging capacity C of the positive electrode is set such that A/C>1 holds, for preventing the deposition of lithium on the surface of the negative electrode.
Further, in Patent Document 2, as an example of a safety measure, there is known a technology of forming an insulating member on a border portion between the application portion and the non-application portion, to prevent the occurrence of a short circuit between the positive electrode and the negative electrode.